A digital switch, as used in telecommunication systems, consists of a plurality of digital switching circuits in an integrated circuit. Signals applied to the inputs of the digital switch are routed to selected outputs in response to control signals.
Digital switches can be used to switch frequency modulated (FM) video information. An analog FM signal is applied directly to an input of the digital switch. Since the switching path within the digital switch includes multiple cascaded digital gates, the analog FM signal is clipped on the positive and negative peaks of the sine wave signal. The output of the switch is a square wave signal having a period which varies in accordance with the input analog FM signal. After filtering of the digital switch output, the analog FM input signal is reproduced.
When multiple FM signals are applied to the inputs of the digital switch, crosstalk results from capacitive coupling between inputs and outputs and from inductive coupling between outputs. The inductive coupling between outputs is caused by stray inductance in the supply voltage and ground lines and can contribute over 90% of the total crosstalk. Reduction of crosstalk caused by inductive coupling would produce a major improvement in FM switching.
It is a general object of the present invention to provide improved methods and apparatus for digital switching.
It is another object of the present invention to reduce crosstalk in digital switching apparatus.
It is a further object of the present invention to provide improved methods and apparatus for digital switching of FM signals.
It is yet another object of the present invention to provide circuits for crosstalk reduction which are simple in construction and low in cost.